The high affinity receptor for IgE on mast cells and basophils (FcEpsilonRI) plays a central role in immediate hypersensitivity reactions. Aggregation of receptor-bound IgE by polyvalent antigen leads to aggregation of the receptors and cellular secretion of both preformed and newly synthesized mediators of inflammation. The molecular mechanisms by which aggregation of the receptors generate these cellular responses are the central focus of our studies. During the past year we focussed on three principal areas: 1) Developing improved methods for discovering proteins other than the subunits of the receptor which might be associated with the receptor either prior to or as a consequence of aggregation. 2) Exploring various aspects of how aggregation may initiate a biochemical cascade, in vitro systems. 3) Devising a procedure that would permit an assessment of the lifetime of activated receptors. 4) Exploring the role of protein kinase C in the phosphorylation of the receptor. Our new results show: 1) Using chemical crosslinking reagents substantial quantities of other cellular proteins (including the src-like kinase lyn) appear to be associated with the receptor. Aggregation of the receptors prior to crosslinking does not lead to substantial differences in the pattern of associated proteins. 2) Conditions have been developed that allow one to observe in vitro a) Substantially enhanced tyrosine kinase activity in receptors that had been aggregated in vivo but only so long as the receptors remain aggregated, b) Evidence that the in vitro phosphorylation of the receptors occurs via a transphosphorylation. 3) Contrary to the proposals of others, aggregated receptors have a substantial half-life of activation. 4) Protein kinase C is implicated in the phosphorylation of the gamma but not of the beta chain of the receptor.